The present invention relates to a method of leaching sulphidic material that contains metals and arsenic under oxidising conditions and with the aid of microorganisms.
1. Field of the Invention
It has long been known to leach sulphidic material, such as ores and ore concentrates, in the presence of microorganisms in the form of different types of bacteria capable of favouring oxidation of both sulphur and iron and other metals in the materials, with the object of winning the valuable metal content of the materials. This type of leaching is also called bioleaching.
For instance, valuable metals can be leached out and brought into solution which is then treated for selective winning of valuable metals, such as copper, nickel, cobalt, uranium and zinc. The noble or precious metal content that cannot be recovered directly by leaching in this way, for instance the precious metal or noble metal content of refractory materials such as iron pyrite and arsenopyrite, can be recovered by first dissolving surrounding metal sulphides so as to free the precious or noble metals, and thereafter treating the bioleaching residue hydrometallurgically in a conventional manner to leach out the precious or noble metals, for instance by treating with cyanide.
Bioleaching processes afford certain advantages over other possible hydrometallurgical processes for working-up metal sulphide material, for instance pressure leaching, by virtue of the fact that bacteria favour oxidation of both sulphidic sulphur and elementary sulphur to form a sulphate. Oxidation of Fe(II) to Fe(III) as well as As(III) to As(V) is also favoured. The bacteria leached material can thus be further leached in subsequent stages, for instance in a precious metal recovery process, without risk of problems caused by the presence of elementary sulphur. However, one serious drawback with bioleaching is that very long leaching times are required at room temperature in order to achieve sufficiently high metal yields. Consequently, it is necessary to work at elevated temperatures and therewith accelerate the leaching process so that leaching can be effected within time periods of reasonable duration.
Bioleaching of different types of sulphidic material with the aid of various types of microorganisms is described in our earlier publication U.S. Pat. No. 5,397,380, while the basic background art in this field may be found in AU-A-11201/92, CA-A-1 023 947 and U.S. Pat. No. 4,571,387, for instance.
In order to accelerate the leaching process and therewith enhance the efficiency of the metal winning process over reasonable leaching times, it is thus necessary to leach at elevated temperatures with the aid of special thermotolerant (thermophile) bacteria cultures, such as those proposed in WO 92/16669 which describes leaching of refractory sulphidic material.
With respect to their ability to resist elevated temperatures, the bacteria cultures concerned can be divided into three groups, namely mesophilic bacteria, e.g. Thiobacillus ferrooxidans, which have a use range of up to 40xc2x0 C. at most, moderate thermophilic bacterial (thermotolerant) that have a use range of up to about 50-55xc2x0 C., and extremely thermophilic bacteria of which some can be used up to a temperature of about 90xc2x0 C., although the majority can only be used effectively at temperatures of 65-70xc2x0 C.
Several investigations in which thermotolerant cultures have been used to bioleach different sulphide materials have been presented in the scientific literature in recent times. For example, E. B. Lindstrxc3x6m et al: J. Ind Microbiol. (1990) 5: 375-382, describe relating to the leaching of arsenopyrite with the aid of extreme thermophilic Sulfolobus cultures, O. H. Tuovinen et al: Appl. Environ. Microbiol. (1994) 60: 3268-3274, describe experiments relating to the leaching of arsenopyrite with mesophilic and moderate thermotolerant bacteria, xc3x85. Sandstrxc3x6m et al: Hydrometallurgy (1997) 46: 181-190, which discuss bioleaching of sulphide ores with the same type of bacteria as the earlier reference, and K. B. Hallberg et al: Appl. Microbiol. Biotechnol. (1996) 45: 212-216, which discuss the toxicity of arsenic in respect of high temperature bioleaching of gold-containing arsenopyrite.
During experiments carried out with the use of extremely thermophilic micro-organisms, for instance of the Sulfolobus metallicus type, it has been established, in several of the aforementioned papers among others, that the possibility of employing bioleaching at elevated temperatures is restricted by the presence of arsenic in the material, since arsenic tends to have a toxic effect on the extremely thermophilic bacteria cultures, although not to the same high extent on the mesophilic and moderate thermophilic cultures, and that this toxicity increases with higher arsenic concentrations in the material. In this respect, As(III) is particularly toxic and, unfortunately, even As(V) exhibits a toxicity which prevents it from being tolerated in large amounts. This toxicity is manifested by the inability of the bacteria to reproduce during the leaching process, which it would otherwise do under conditions favourable to reproduction, and is therefore never effective. Consequently, in order to make such bioleaching of arsenic containing material possible, it is necessary to heavily dilute the pulp concentration, which can also be expressed as pulp density, i.e. the ratio of material quantity to leaching solution volume must be kept low so as to fall beneath the toxic limit of the culture concerned with respect to arsenic. It will be understood that this problem has, unfortunately, a highly negative effect on the economy of the bioleaching process with respect to working-up arsenic containing materials.
An object of the present invention is to eliminate the problems that the toxicity of arsenic creates with respect to extremely thermophilic bacterial cultures, so that an economically attractive process can be provided for bioleaching arsenic-containing sulphidic ores and concentrates of such ores.
This object is achieved by the present invention, which comprises the features and stages specified in the accompanying Claims.